The present invention relates to a method for finishing a game ball which has a primer covered by a polyurethane top coat forming the outer surface of the ball. The invention particularly pertains to a method for finishing a golf ball.
A primer is used as an interface between the cover and top coat of a golf ball to improve the adhesion between them. In the past, such primers have included from about 1% to about 3% by weight of a polyfunctional aziridine crosslinker. These proportions of the crosslinker are the crosslinker manufacturer's recommendations to promote adhesion of the primer to substrates. U.S. Pat. No. 4,278,578 ("the '578 patent"), issued to Carpenter, teaches latices containing polyfunctional aziridines. The '578 patent indicates that the use of more than 3% aziridine is not beneficial to coating properties (column 3, lines 21-25).
The present inventors have discovered that the degree of adhesion promoted by the recommended amounts of the polyaziridine cross-linker is not optimal in a primer for adhering the polyurethane top coat to the thermoplastic ionomer resin materials of the type used in golf ball cover compositions.
One complicating factor respecting the use of a primer to join the cover and the polyurethane top coat of a golf ball is that golf balls are intended to be used outdoors under severe conditions at a wide variety of temperatures. Golf balls must be capable of withstanding sunlight and occasional immersion in water traps or puddles, and preferably should be resistant to extended immersion in water, as when they are lost in a water trap, recovered later, and ultimately returned to service. Golf balls are severely jarred, and thus flexed, every time they are hit with a golf club.
Golf balls are not regarded as perishable. They are not stored at a controlled temperature or humidity, and thus can be exposed to wide variations in ambient conditions for a substantial period before they are used, even for the first time. All these conditions require superior adhesion of the polyurethane top coat to the cover of the ball, and yet make it difficult to maintain even adequate adhesion.